Primate Dads Gain Sympathy Weight

Between 11 and 65 percent of all human dads-to-be experience sympathy symptoms when their partners are pregnant, including weight gain, nausea, headaches, irritability, backaches, and hormonal changes. Now a new study has found that males in at least two species of primates also experience physiologic changes when their mates are expecting.

Researchers at the University of Wisconsin, Madison, observed that two types of primates, the common marmoset (Callithrix jacchus) and cotton-top tamarin (Saguinus oedipus) - both known for their monogamous partnerships and good parenting - gained roughly 10 percent more weight when their mates were pregnant. And scientists believe they're not alone; it is estimated that males in most monogamous primate species - including gibbons, some lemur species, and humans - experience such sympathy symptoms.

During the study, researchers weighed 29 male marmosets and 29 male cotton-top tamarins monthly. Of these, 9 marmosets and 11 tamarins had pregnant mates. The scientists found that the males increased their total weight by approximately 10 percent gradually over the course of their mate's pregnancy (five months for marmosets and about six months for tamarins). The researchers also determined that the added weight was not because the males copied their mates' eating habits.

Scientists believe that the primates are so in tune with their mates that they pick up subtle clues about their fertility. According to Toni Ziegler, a senior scientist at the University of Wisconsin, Madison and lead author of the study, "They…detect when their mate is going to ovulate and have an increase in testosterone a few days before she ovulates, so I think they are getting signals from the female at important reproductive events." The researchers observed elevated estrogen and testosterone levels in the males, as well as the lactation-inducing hormone prolactin which they believe is most likely the cause of the weight gain.

These pregnancy symptoms were previously thought to be psychosomatic, but scientists now believe that these changes may actually help the male primates to prepare for fatherhood and deal with the added stress once the baby is born. "Males do most of the carrying of infants - usually two - once they are born," said Zigler, adding, "The males invest highly in infant care, even losing weight while carrying these heavy, multiple infants through the trees."

These findings are giving scientists a window into the driving force behind fatherhood - primate and human. "We're interested in what motivates dads to be good parents because there are so many men who just aren't good fathers," says Ziegler. "This work could help to tease apart what makes a good dad."